Re: [next] i386: kunit: ASSERTION FAILED at mm/kfence/kfence_test.c:547

Marco Elver
2022-05-02T07:51:56Z

On Sat, Apr 30, 2022 at 11:14PM +0206, John Ogness wrote:
[...]
> I am not familiar with how this works. Is the tracepoint getting set on
> call_console_drivers()? Or on call_console_driver()?

It's at the start of call_console_drivers(). See trace_console_rcuidle()
call.

> If so, there are a couple problems with that. First off, the prototype
> for that function has changed. Second, that function is called when text
> is printed, but this is not when the text was created. With the
> kthreads, the printing can be significantly delayed.
>
> Since printk() is now lockless and console printing is delayed, it
> becomes a bit tricky to parse the records in the existing code using a
> tracepoint.
>
> I wonder if creating a NOP function for the kfence probe to attach to
> would be more appropriate. In printk_sprint() we get the text after
> space has been reserved, but before the text is committed to the
> ringbuffer. This is guaranteed to be called from within the printk()
> context.

I think we just need to fix the existing tracepoint, since it has
changed its semantics vs. what it was intended to be:

| commit 95100358491abaa2e9a5483811370059bbca4645
| Author: Johannes Berg <johannes.berg@intel.com>
| Date: Thu Nov 24 20:03:08 2011 +0100
|
| printk/tracing: Add console output tracing
|
| Add a printk.console trace point to record any printk
| messages into the trace, regardless of the current
| console loglevel. This can help correlate (existing)
| printk debugging with other tracing.
|
| Link: http://lkml.kernel.org/r/1322161388.5366.54.camel@jlt3.sipsolutions.net
|
| Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
| Cc: Christoph Hellwig <hch@infradead.org>
| Cc: Ingo Molnar <mingo@redhat.com>
| Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
| Acked-by: Thomas Gleixner <tglx@linutronix.de>
| Signed-off-by: Johannes Berg <johannes.berg@intel.com>
| Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

Specifically using it to "correlate (existing) printk debugging with
other tracing" is now broken.

> Here is an example of what I am thinking...
>
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2227,6 +2227,10 @@ static u16 printk_sprint(char *text, u16 size, int facility,
> }
> }
>
> +#ifdef CONFIG_KFENCE_KUNIT_TEST
> + printk_kfence_check(text, text_len);
> +#endif
> +
> return text_len;
> }
>
> The probe_console() could attach to a NOP function printk_kfence_check().

Thanks for this! However, I think we can't have a KFENCE-specific
helper, it needs to be a tracepoint, because there are more tests that
want to check console output (kernel/kcsan/kcsan_test.c did this before
the KFENCE test actually).

My proposal would be to fix the tracepoint like so:

| --- a/kernel/printk/printk.c
| +++ b/kernel/printk/printk.c
| @@ -2002,8 +2002,6 @@ static void call_console_driver(struct console *con, const char *text, size_t le
| {
| size_t dropped_len;
|
| - trace_console_rcuidle(text, len);
| -
| if (con->dropped && dropped_text) {
| dropped_len = snprintf(dropped_text, DROPPED_TEXT_MAX,
| "** %lu printk messages dropped **\n",
| @@ -2178,6 +2176,8 @@ static u16 printk_sprint(char *text, u16 size, int facility,
| }
| }
|
| + trace_console_rcuidle(text, text_len);
| +
| return text_len;
| }

This fixes the KFENCE and KCSAN tests.

Unless I hear objections, I'll prepare a patch explaining why we need to
fix the tracepoint.

Thanks,
-- Marco